Conventional internal combustion engines are constructed so that the engine crank shaft, pistons, valve cam shaft and valves are linked together mechanically. In such engines, valve opening and closing is "timed" with respect to the position of the piston with which the valve is associated.
Timing of a valve is by a somewhat teardrop-shaped cam mounted on a rotating cam shaft. That portion of the cam protruding outward from the otherwise-circular shape of the cam is called the cam lobe. The lobe is that portion of the cam which directly or indirectly opens a valve once for each cam revolution. As the cam rotates, valve closure is by a compressed spring. Absent a control arrangement (and very simplistically), valve timing can be modified in either or both of two ways--and both require engine disassembly. One is by substituting a cam with another having a differently-shaped lobe. Another is by repositioning the cam on its axis of rotation so the lobe opens the valve earlier or later than normal.
So-called "normal" valve timing is generally satisfactory over a relatively wide range of speed and load operating conditions. Nevertheless, it is a compromise for many engine operating conditions. Stated another way, the ability to control timing helps optimize performance under such conditions. Such timing control may make possible improved efficiency, higher torque, reduced fuel consumption and reduced emissions.
In view of the prospective advantages, designers in this field have configured arrangements for changing valve timing while the engine is running. Examples of such arrangements are shown in UK Patent Application 2 144 176 (Doorbar); U.S. Pat. No. 4,205,634 (Tourtelot, Jr.); German Patent Document DE 3406100 (Roth) and UK Patent Specification 1 496 513 (Scarrot).
The arrangement shown in the Doorbar UK patent document involves planar or contoured inserts introduced between the cam and the cam follower. Such inserts are said to change both timing and lifting pattern, i.e., the distance to which a valve opens.
The arrangement shown in the Tourtelot, Jr. patent involves a movable wedge inserted between the valve tappet and the lobes of two cams. Using such arrangement, the time at which a valve starts to open can be varied as a function of the instantaneous position of the piston and crank shaft. However, the distance to which the valve opens must also change since the thickness of material interposed between a cam lobe and the valve tappet varies with changes in wedge position. To put it another way, there can be operating conditions where a cam lobe does not contact the top surface of the tappet during an entire cam revolution.
The Roth patent document shows a movable insert having a control surface used to affect the time at which a valve opens. The component on which the control surface is formed is movable toward and away from the cam by a crank-like actuator. In the illustrated arrangement, the insert may depress the tappet eccentrically rather then concentrically. FIG. 2 shows such eccentricity in that the projection from the bottom of the insert is not coincident with the valve centerline.
And the intermediate position of the insert shown in FIG. 2 (the crank-like mechanism is at neither extreme of travel) suggests there are at least some positions of such insert where the valve may be opened farther than normal rather than merely earlier or later than normal. In other words, if the insert is moved to its extreme right position, the thickness of the insert at the valve centerline is increased and the distance of valve opening will be increased.
The arrangement shown in the Scarrot patent document involves what is called an interposer, i.e., a component to modify valve timing. Such interposer is positioned by a relatively complex eccentric crank mechanism and is said to provide independent control of valve opening and closing times.
As explained, arrangements like those shown in the Tourtelot Jr. and Doorbar patent documents affect the dimension to which a valve is opened. Protrusion of the valve head further into the piston chamber and further compression of the valve spring beyond that occurring during normal timing can present two problems. One is that the protruding valve head may be struck by the piston. If this occurs, damage to the piston and/or the valve is virtually certain. And the increased area of opening between the valve head and the engine block may cause a modification in the fuel/air charge introduced into the piston chamber.
Excessive compression could unduly stress the spring and bring on premature spring failure. And if the spring is compressed to its solid height, other parts in the valve train could be damaged.
In those and other known arrangements, no provision is made to bring the control insert to a "home" position in event of failure of an aspect of the control system. Absent such provision, the insert may come to a random position which may or may not be acceptable for the then-prevailing engine operating condition. Worse is the possibility that the insert may come to position at which the engine will not run at all under normal operating conditions.
Yet another disadvantage of known arrangement is that valve operating and control components are made (or are apparently made) of steel formulated for such applications. Such steel is relatively heavy and has significant inertia, a factor in setting the maximum speed of engine operation.